1. Field of the Invention
The present invention relates to a piezoelectric element having an unstressed piezoelectric film, and a gyroscope that uses the piezoelectric element.
2. Description of the Related Art
It is known that properties of a piezoelectric film such as a permittivity, a Curie temperature, a coercive electric field, and a residual polarization vary according to an internal stress of the piezoelectric film. A piezoelectric film formed by a thin film process tends to have a two-dimensional stress that occurs in a film surface during film formation. Accordingly, to obtain a piezoelectric element exhibiting excellent piezoelectric properties, development of a piezoelectric film having a crystal structure with a reduced internal stress is desired. As a piezoelectric element having a crystal structure for reducing an internal stress, for example, Japanese Patent Application Laid-Open No. 2008-28285 proposes a piezoelectric element that has a laminate structure formed by stacking first, second, and third piezoelectric films from one electrode toward another electrode, where a piezoelectric constant of the first and third piezoelectric films is smaller than a piezoelectric constant of the second piezoelectric film. Such a device structure allows for a reduction in internal stress that occurs in an interface between an electrode and a piezoelectric film.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-28285
However, in the device structure disclosed in Japanese Patent Application Laid-Open No. 2008-28285, a difference in physical property such as a lattice constant, a thermal expansion coefficient, and the like between the piezoelectric film and its base may cause a compressive stress or a tensile stress to occur in the piezoelectric film during a process of cooling the piezoelectric film after high-temperature film formation. This makes it difficult to achieve a sufficient stress reduction.